1. Technical Field
The invention relates to a process for improving furnace operation, and in particular, for reducing, eliminating or modifying: slagging, convective tube fouling, fireside corrosion, fly ash, sulphur trioxide formation, acid smut, and plume visibility problems.
2. Discussion of Prior Art
The recent widespread shutdown of nuclear power generators and the continued instability in both the supply and price of natural gas places an additional burden on coal and oil burning power plants to provide dependable low to medium cost power. This shift has added impetus to research directed towards improving power plant cost effectiveness, environmental cleanliness and fuel efficiency.
One aspect of research has focused on controlling fireside combustion by-products, as they have a tremendous effect on the efficiency and cleanliness of boilers as well as the costs of fuel that they can burn.
Convective tube fouling which results from liquid or sticky phase ash deposition on the tubes, can affect power plant efficiency and cleanliness. Ash deposition is attributed to the fact that the low melting point of coal and oil ash (between 1,000.degree. and 1,200.degree. F.) is often the operating temperature of power plant superheater and reheater tubes. As a result, the ash enters a liquid or sticky phase forming deposits along the nearest surfaces which become harder as the boiler continues to operate. These deposits "foul" areas of the boiler, particularly the superheater tubes, and reduce plant cost effectiveness by reducing the thermal exchange between the superheater tubes and the steam passages. This results in frequent boiler down times for cleaning. This problem has restricted plants to burning higher priced fuel that tend to be low in sodium, sulphur and vanadates. It would be desirable, therefore, to be able to eliminate the liquid or sticky phase which occurs during fly ash deposition.
Fly ash, produced in the furnace during combustion and transported through the superheater tubes, also contributes to the corrosion of tube surfaces as a result of the formation of molten ash deposits. In a paper entitled "Fuel Ash Corrosion and its Effect on Boiler Design", by Hansen et al., Transactions of ASME; April 1965, pp. 210-214, industry-reported increases in oil ash and gas-side corrosion were attributed to the increase in gas and/or metal temperatures in new generation boilers; high vanadium content oil; and liquid phase alkali-sulfate compounds which contact the hot metal surfaces. Corrosion is a major deterrent to the effectiveness of newer high temperature boilers as corrosion cannot be prevented merely by the use of high cost, high strength metals. Consequently, avoiding corrosion means reducing fouling deposits and, therefore, controlling the deposition of liquid or sticky phase fly ash.
Slag deposits usually form in the low velocity portions of the boiler passages. The formation of slag results from deposition of heavy waste and combustion by-products in a liquid phase. Control of slagging is a key to enabling the use of refuse-derived and other low cost fuel substitutes as slag formation and the hardness characteristics of slag deposits are a function of the concentration of glass in the fuel. Refuse-derived fuels, for example, are known to contain up to 20% glass. Removal of slag deposits is extremely time-consuming as it requires operation stoppage and steam cleaning and/or air lancing the affected boiler areas. In many cases, slag removal operations can be extremely costly. Heavily slagged checker chambers in open-hearth furnaces, for example, have to be dismantled requiring replacement of as many as 30,000 to 40,000 checker bricks. In this case furnace shutdown time can become severe.
Another long recognized problem in boilers relates to the presence of sulfur trioxide. Sulfur trioxide is formed through conversion in the boiler gas passages of sulfur dioxide to sulfur trioxide with fly ash acting as the catalyst. Sulfur trioxide tends to condense in the cooler sections of the heat exchangers in the form of sulfuric acid. The acid participates in the corrosion of the heat transfer surfaces as well as reducing and limiting the heat economy obtainable. Furthermore, in their paper entitled "Corrosion of Superheaters and Reheaters of Pulverized Coal-Fired Boilers", Melson et al., Journal of Engineering for Power; Transactions of ASME 1960 p. 194 strongly link the presence of sulfur trioxide to heavy ash deposits. The authors report that ash deposits have a layered structure, the outermost portion of which comprises a friable fly ash layer which is formed by the mineral portions of the coal and its sulfurous reaction products. The inner layer comprises a harder material containing substantial amounts of sulfur trioxide which, when mixed with the alkaline earth oxides in the fly ash, form complex sulphates. The authors conclude that the complex sulfates are the principal molten compound that bonds the ash deposits to the tube walls and are also the principal components in corroding the tube. Reduction or elimination of sulfur trioxide formation, therefore, is critical to maintaining the boilers in good operating condition. Control of corrosion avoids the necessity of frequent equipment replacement and periodic shutdowns.
Finally, boiler emissions are a major environmental concern, particularly with the respect to less direct and obvious forms of pollution, such as acid rain. Visible emissions have been generally reduced by employing scrubbers and other costly mechanical systems. However, the acid content of these emissions must also be reduced. Therefore, the need to control both the visible plume as well as the acid content of the emissions is critical.
In developing a process for reducing, eliminating or modifying corrosion, fouling, slagging, sulfur trioxide, fly ash, acid smut and plume visibility, an effective, simple and efficient process using comparatively inexpensive materials that can rapidly reduce these problems is highly desirable.
The primary existing method for controlling the above-described fireside related problems has been the addition of chemical additives which raise the melting point of the fly ash, resulting in more friable tube deposits and slag as well as restricted sulfur trioxide formation, reduced corrosion, acid smut and plumes. However, these prior additive addition and treatment techniques have not employed a single additive which, by virtue of an economic injection process, reduces all of the above conditions and requires only minimum quantities of the additive. Further, many prior art additives have created other problems, some of which exceed the problems they solve.
In a paper "Slurry Spraying for the Control of Corrosion and Deposits in Oil Fired Boilers", Cantieri et al., ASME paper No. 60-W-284, presented at the 1960 Annual Winter Meeting, New York, the authors review research on additives and their method of injection into oil-fired boilers in order to control a number of fireside-related problems.
Harlow ("Formation of Sulfuric Acid in Boiler Flue Gases", Transactions of ASME, 1958 p. 225), for example, describes spraying calcium oxide on the boiler tubes in order to inhibit catalytic oxidation of sulfur dioxide into sulfur trioxide. However, calcium oxide inhibited the reaction for only several hours. The authors also report that Rendle et al., "The Prevention of Acid Condensation in Oil-Fired Boilers", Journal of the Institute of Fuel, 1956, pp. 372-380, found that magnesium oxide, zinc dust, dolomite, and gaseous ammonia eliminated the acid dew point, i.e., the point at which sulfur oxide is catalyzed into sulfur trioxide.
With respect to fouling and slagging, Cantieri et al. report that Keck, "Retarding Corrosion and Deposits of the Fire-Side Surfaces of Boilers Fired with Residual Fuel Oils", presented at the Southeastern Electric Exchange, 1959, found that additives of dolomite, high-magnesium lime, magnesium oxide and lime had varying effects on reducing these deposits. Keck also found that the chemicals and, in particular dolomite, slightly reduced the volume of slag. However, the treatment described left tons of additive, vanadium compounds and slag boulders on the furnace floor as a result.
Cantieri et al. then describe a method for slurry spraying the boilers through the soot blowers. The method involves removing loose deposits by first operating the blowers at full pressure. Blowing pressure is then reduced by 50 psi and a slurry consisting of calcium oxide and magnesium oxide particles suspended in an aqueous solution is then introduced along with the blowing medium. Subsequently, the slurry heads are purged with water to properly clean them. The spray system is usually energized once a day. As a result, fouling, corrosion, plume visibility and acid dew point problems are reduced or eliminated. However, the authors report that the degree of improvement appears to be most marked after the additive has been applied and deposit formation continues to occur between the daily injections.
A number of patents also describe processes using additives to control boiler related problems. Chauhan et al., U.S. Pat. No. 4,280,817, is typical of these patents wherein a solid fuel, such as coal, is treated with a catalytic agent so that the coal is physically and chemically altered. The coal is then catalyzed in order that the incorporating catalyst acts as a sulfur absorbent during combustion. The fuel is treated in a liquid medium containing both calcium oxide and magnesium oxide. The slurry is then subjected to elevated temperatures and pressures such that the catalytic agent physically incorporates the water and fuel. The agent comprises either calcium hydroxide, magnesium hydroxide or a possible combination of both. Chauhan et al., however, do not disclose treating the fuel for prevention of corrosion, fouling, slagging, etc. and do not disclose a method that employs small quantities of an additive to alleviate sulfurous emissions.
Other patents which treat one or more boiler problems by adding chemical additives to the boiler include: U.S. Pat. No. 4,185,080, U.S. Pat. No. 3,249,075, U.S. Pat. No. 3,002,855, and U.S. Pat. No. 3,919,394. A large number of additives in these patents and other prior art have been proposed. Based on their chemical constituents and physical characteristics, such additives are:
MgO (oil dispersion) PA1 CaO (dry) PA1 MgO+Al.sub.2 O.sub.3 (oil dispersion) PA1 MgO+Mg(OH).sub.2 (dry) PA1 Oil-soluble Mg (magnesium naphthenate, etc.) PA1 Oil-soluble MgO+MnO PA1 Mn (Oil Soluble) PA1 CaCo.sub.3 PA1 MgO+CaO (aqueous dispersion) PA1 Dolomite
In summary, therefore, while prior art techniques illustrate a host of additives that are effective in solving a host of specific boiler and fireside related problems, a single additive composition that effectively solves all of the previously described problems has not been found. Additionally, the processes and techniques for introducing these additives into the boiler have not resulted in efficient use of the additives such that material is wasted and additional time is required to reduce these fireside-related problems.
In our previous patent, U.S. Pat. No. 4,246,245, a process for removing sulfur dioxide from boiler effluent gases is described. The process involves contacting the gas in a wet or dry scrubbing zone with recycled tank slurry that contains Type S hydrated dolomitic lime.
However, it was not previously appreciated that the additive composition is highly effective in reducing, eliminating or modifying fouling, slagging, corrosion, sulfur trioxide formation, fly ash, acid smut and visible plumes. In addition, it was not recognized that a more efficient and more effective technique for injecting the additive into the system was available over the continuous method of injection described in U.S. Pat. No. 4,246,245. Finally, it was not appreciated that the injection technique could direct the additive to specific problem sites, rather than generally throughout the boiler interior.